1. Field of the Invention
This invention relates to filtration of solids from water and more particularly to filter systems for fish culture tanks.
2. Description of the Related Art
In the culture of fish in small quantities, such as in aquarium culture, solid wastes are removed by capturing the solids in filters that must be cleaned by hand after they have become loaded with solids. Although labor intensive, this method is the preferred for small culture systems.
In large outdoor fish farms, such as catfish farms, the solids collect on the bottoms of ponds and are removed once per year after the fish have been harvested. This process results in all of the water in the pond being discarded (approximately 1-5 million gallons) and requires that new water be supplied before new fish can be added to the pond. The pond draining, drying, and refilling process can take as much as a month to accomplish.
Where fish are grown in raceways, such as in trout farms, solids are collected in areas near the end of the raceways and drained off to settling ponds. These ponds may be as large as one quarter the area of the raceways. These settling ponds must also be drained and cleaned by hand once or twice per year.
In Europe where Atlantic salmon are grown, some hatcheries are indoors using heated water for optimum growth. As settling ponds are too large to be placed indoors, a form of mechanical micro-strainer is used to remove solids. The fine screens used are cleaned by jets of high pressure water applied to the clean side of the screen which removes the solids but adds water. The ratio of water-in to water-out is about 100:1 and with a water flow of 24,000 gallons in per hour, the solids-rich waste water generated in this method typically amounts to 240 gallons. This solids-rich waste water cannot be discharged to a stream but must be collected and stored to be treated later by yet another process.
In a typical commercial fish growing operation, fish are stocked at about one pound per gallon of water. In a system containing 12,000 gallons of water, 12,000 pounds of fish can be fed three percent of body weight per day or 360 pounds per day or 30 pounds per hour dry weight. The wet weight of feces and uneaten food is about equal to the dry weight of food fed.
Efforts to avoid the generation of large quantities of waste-laden liquid during the cleaning process have been generally unsatisfactory in the fish farming industry. However, in an unrelated art area, U.S. Pat. No. 4,772,398 dated Sep. 20, 1988 shows a means of removing textile fibers from a stream of water containing fibers that have been concentrated by aeration of the incoming stream. The fibers are concentrated and are picked up by the belt and vacuumed off the belt and recovered.
The inapplicability of this scheme shown in the '398 patent to the fish farming tanks is evident in the patent's use of a belt as a waste pick-up. In fish farming operations, waste must be filtered out by a flow-through system thereby allowing removal of much smaller particles and also allowing particle removal at lower concentrations of waste per volume of liquid.